Yesterday, my manager told me that my colleague would get fired. Then I asked my boss for the reason, and he gave an explanation. After we finished the conversation, I continued working. When I came back home, I couldn't stop thinking about it.

My boss didn't say (explicitly) that I couldn't tell him. Is it advisable to tell my colleague that he is getting fired?

And to add: what your boss did was not very wise. If person X gets fired, X should be the first to know, not his colleagues. This leads to all kinds of awkward situations, as your question shows.
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Jan DoggenApr 19 '14 at 19:28

3

Also, there is the very human tendency of human beings to "shoot the messenger". Even though you had nothing to do with why this person was fired, by being, perhaps, the first person to tell him, you associate yourself with his being fired and any emotions that he feels about being fired may redirect themselves to you.
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Francine DeGrood TaylorApr 21 '14 at 16:10

To echo the answers: it's absolutely not advisable. The more troubling question here is: why did your boss tell you about this? Typically any internal communication about an impending layoff is big no-no, at least in any reasonably sized US company with an at least half-way functional HR department. In some companies I've worked for this would be considered a fairly serious transgression.

So it's possible that your manager has some sort of non-obvious agenda in telling you this. It may be helpful to cautiously ping him on this. For example

"Hey Mr Boss. Thank you very much sharing this very important information with
me. I assume that this is still confidential at the moment, so I'm
not going to share this with anyone unless you tell me it's okay to do
so. I was also wondering whether there are any specific things you'd
like me to do differently or specific actions you expect from to help with the situation".

Something along these lines. It's basically a diplomatic way of saying "Why the heck did you tell me that and what do you want me to do with it?" It may be also helpful to keep detailed records on any conversation with your manager on this topic. If something fishy is going on, your records can help in demonstrating that you have acted transparently and with integrity.

One reason someone would tell you this is because you 'need to know', particularly if you have to be prepared to mop up something left by the now departed colleague. This could be backing up a server or changing locks on a door or securing a storage area - who knows. It might also be necessary for you to take over a customer relationship that the other party has, you might need to know this if the customer calls.

If your boss provided an explanation, it's quite possible you're being invited to drop a hint: for your colleague to clean up his/her act. Since you haven't indicated whether it was a performance issue or a resources issue (i.e., a layoff due to cutbacks) it's hard to tell why this is being done. Realistically, dropping hints to people that they're not performing is useless - it isn't likely to affect behavior, other than to, perhaps, give them an opportunity to mess something up on their way out.

If your boss was engaging in somewhat of a dirty trick, he might have you tell your co-worker that he's being fired in order to wreck his morale, triggering even more performance issues. This seems rather dangerous, and if this is what your boss was doing, he wouldn't answer your question in so many words.

Also, if he was worried that people would be upset about the firing, he might want to be proactive in addressing it. I recently had a boss come to me and say that a coworker had been let go. He explained his reasons and assured me (and the two other coworkers present) that nobody else was being let go and the project was not in jeopardy. It sounds as if your boss might have meant for you to be reassured when you found out about the firing, although I can't imagine why he would tell you about it before completing the process.
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Francine DeGrood TaylorApr 21 '14 at 16:16